This invention relates to photosensitive elastomeric elements having a layer of photosensitive, solvent-soluble, thermoplastic, elastomeric compositions which upon exposure to actinic light form solvent-insoluble, elastomeric materials, and to a process for making reliefs from such elements, and still more particularly, to a process for making flexographic printing reliefs from such elements.
Flexographic printing reliefs which are useful for letterpress printing of packaging materials, e.g., cardboard, plastic films, etc., are conventionally prepared by a laborious procedure involving art work, photographic negative, photoengraving, formation of a phenolic matrix, and hot press molding of a rubber plate. Printing reliefs with resilient surfaces can be prepared directly from photosensitive compositions as exemplified in assignee's U.S. Pat. Nos., McGraw 3,024,180, granted Mar. 6, 1962 and Barney 2,948,611, granted Aug. 9, 1960. In the processes described in said patents, printing reliefs having characters of uniform printing height are produced by exposing to actinic light through an image-bearing transparency (negative or positive) a layer comprising an addition-polymerizable, ethylenically unsaturated compound or mixture of compounds, a solvent-soluble elastomeric binder, and having dispersed therethrough an addition-polymerization initiator activatable by said actinic light, and being superposed on and adherent to a suitable support, e.g., a metal plate or foil, until addition polymerization, i.e., insolubilization of the composition occurs in the nonexposed areas. Removal of the layer in the nonexposed areas, e.g., by treatment with a suitable solvent in which the polymerized composition in the exposed areas is insoluble, leaves a printing relief of the line or halftone image of the transparency suitable for letterpress work. Frequently, however, these printing reliefs are not as elastomeric as the molded rubber plates. Also the photosensitive elements tend to cold flow due to high monomer concentrations needed and do not have the desirable features of vulcanized rubber without reducing solvent solubility as well. Furthermore, such compositions and elements often must be solvent coated, thus reducing the effective use of the elements.